Re: [PATCH 2/2] mm: add a field to store names for private anonymous memory

[Colin Cross <ccross@android.com>]

On Sun, Jul 14, 2013 at 7:11 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> Sorry if this was already discussed... I am still trying to think if
> we can make a simpler patch.
>
> So, iiuc, the main problem is that if you want to track a vma you need
> to prevent the merging with other vma's.
>
> Question: is it important that vma's with the same vma_name should be
> _merged_ automatically?
>
> If not, can't we make "do not merge" a separate feature and then add
> vma_name?
>
> IOW, please forget about vma_name for the moment. Can't we start with
> the trivial patch below? It simply adds the new vm flag which blocks
> the merging, and MADV_ to set/clear it.
>
> Yes, this is more limited. Once you set VM_TAINTED this vma is always
> isolated. If you unmap a page in this vma, you create 2 isolated vma's.
> If, for example, you do MADV_DONTFORK + MADV_DOFORK inside the tainted
> vma, you will have 2 adjacent VM_TAINTED vma's with the same flags after
> that. But you can do MADV_UNTAINT + MADV_TAINT again if you want to
> merge them back. And perhaps this feature is useful even without the
> naming. And perhaps we can also add MAP_TAINTED.
>
> Now about vma_name. In this case PR_SET_VMA or MADV_NAME should simply
> set/overwrite vma_name and nothing else, no need to do merge/split vma.
>
> And if we add MAP_TAINTED, MAP_ANONYMOUS can reuse pgoff as vma_name
> (we only need a simple changes in do_mmap_pgoff and mmap_region). But
> this is minor.
>
> Or this is too simple/ugly? Probably yes, this means that an allocator
> which simply does a lot of MAP_ANONYMOUS + MADV_TAINT will create more
> vma's than it needs. So I won't insist but I'd like to ask anyway.

This is no different than using a new tmpfs file for every mmap
(although it saves the struct file and the inode), it results in a
huge increase in the number of vmas.

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